Jian Fu from Shanghai Observatory is visiting
ATNF this week. He works on semi-analytical models of galaxies under a Lambda-CDM cosmology (see Fu et al. 2010). One advantage of his model is to deal with HI - H2 transition in the process rather than in a post-process way. The figure shows his recent work on the HI size-mass relation of galaxies (see for recent observational work of the relation). He compared the models (blue dots) to various observational data sets. The left and right panels are for different HI - H2 transition prescriptions in the model: on the left the transition is regulated by gas surface density and metallicity and on the right the transition is regulated by interstellar pressure. On average the simulation and data agree well. However, there is a noticeable increase of scatter at the low mass end in both panels, which is not observed with real data. More work is in progress in collaboration with Jing Wang to understand and fix the discrepancy and finally understand the relation.